Electrochemical method, electrolyte therefor, and method for producing said electrolyte



ELECTROCHEMICAL METHOD, ELECTROLYTE THEREFOR, AND METHOD FOR PRODUCING SAID ELECTROLYTE Elizabeth M. Zimmerman, Delaware County, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corpor'ation of Pennsylvania J No Drawing. Application January 6, 1958 t Serial No. 707,112

Claims. (Cl. 20414) My invention relates to an electrochemical method for electrodepositing simultaneously the metals indium and gallium, to an electrolytic solution which is especially well suited for use as the electrolyte in my novel electrochemical method, and to a novel method for producing this electrolyte.

Because of its importance in the fabrication of certain types of transistors described hereinafter, a problem for which the electrochemical art currently seeks a solution is how to codeposit electrolytically indium and gallium, metals which have similar'chemical properties but dissimilar singledeposition potentials. Heretofore when the art was faced with such a problem it frequently solved it by preparinga bath composed of the salts of the metals to be" plated dissolved in a suitable polar solvent, such as water, and also importantly containing a substance tending to complex to a greater extent the metal of lower deposion potential. Under these conditions, the effective deposition potentials of the two metals would frequently be brought sufiiciently close together that both could be codeposited at a reasonable current density by apply ing between anode and cathode a single value of voltage.

However this solution has been found useless for the purpose of codepositing indium and gallium. Specifically, because of the chemical similarities between indium and gallium, the available complexing agents tend to combine with the ions of each of these elements to substantially the same extent. As a result, the plating ,characteristic of the solution containing the complexing agent is substantially the same as that displayed by a solution in which this agent is omitted. However, the latter solution is unusable for practical purposes to codeposit indium and gallium because the polarization curve of indium, which plates at the lower deposition potential, rises steeply after this deposition potential has been attained. By

reasonof this polarization. characteristic displayed by.

indium, the current density in an electrolytic solution con taining indium and gallium ions may rise to extremely high values, e.g. many'hundred amperes per square centimeter, where, in an attempt to codeposit the gallium with the indium, the potential applied between anode and cathode .is' increased by even a small amount above the deposition potential of indium. Moreover, under these conditions, even if any gallium is codeposited, the amount deposited is negligibly small compared to the amount of indium simultaneously codeposited, because substantially allof the current through the electrolyte is transported by the indium ions. Accordingly, an electrolyte prepared in accordance with the prior art by dissolving indium and gallium salts in a polar solvent such as water, even when it contains a complexing agent, is substantially useless for the electrolytic codeposition of indium and gallium therefrom.

Moreover, it has been found that indium-gallium alloys should preferably be deposited in a molten state,- thereby to obtain a dense, adherent, smooth plating free of 2,931,758 Patented Apr. 5, 196i) 2 are plated below their melting points. Because indiumgallium alloys may have melting pointsin the neighborhood of the melting point of indium, i.e. about C., water is an unsuitable solvent for use in an indiumgallium plating bath since the plating temperature far exceeds its boiling point.

With the invention of the microalloy transistor described and claimed by A. D. Rittman in his co-pending application Serial No. 585,670, filed May 18, 1956, and now Patent No. 2,870,052, and entitled Semiconductive Device and Method for the Fabrication Thereof," and especially in view of the invention by R. A. Williams of his process for'manufacturing such transistors, described and claimed in his co-pending application Serial No. 582,723, filed -May 4, 1956, and entitled Method for Fabricating Semiconductive Devices, both of which applications are assigned to the assignee of the present application, the problem of providing suitable electrolytes and methods for codepositing indium and gallium on a conductive element has assumed considerable importance.

Specifically, and as described by both inventors in their above-identified applications, such transistors characteristically comprise a semiconductive body having a thinassociated with microalloy contacts in turn causes the' transistor'embodying them to exhibit a high current ef ficiency, Le. a high value of beta. In addition the fact that the'surface of the microalloy contactfaithfully follows the surface of the body permits advantageous shaping of the rectifying contact by use of the electrochemical etching of the body, while the extreme thinness of the contact permits it to be employed in transistors having very thin base regions, thereby making it possible to fabricate transistors which amplify efficiently even at high frequencies.

In the above-identified Williams application, a preferred method for fabricating microalloy rectifying contacts and for simultaneously bonding a wire lead thereto is described. According to Williams method in one preferred form, an indium coating is applied, e.g. by jet electrodeposition, over the surface region of an n-type germanium body beneath which the microalloy rectifying contact is to be formed. Then, as an important feature of the method, a wire lead having electroplated thereon a globule of a solder composed of indium containing a few tenths of a percent of gallium is abutted against the indium coating on the germanium body, and the indiumgallium solder is heated carefully just to its melting point. As soon as the solder has melted, the heating is discontinued and the solder cooled below its melting point. This brief heating suflices to admix the solder with the indium plating below it, to cause a large percentage of the gallium contained by the solder to alloy into the germanium body to a very small depth, thereby to form the microalloy rectifying contact, and to bond the wire lead to this contact.

From the foregoing it will be apparent that an important step in the successful commercial practice of this Williams process is the electroplating onto the wire lead of an indium-gallium alloy containing a controlled quantity of the order of a few tenths of a percent of gallium.

Accordingly it is an object of my invention to provide an improved method for electroplating an indium-gallium alloy onto a conductive element.

Another object is to prepare a novel electrolyte which is especially suitable for use in electroplating an indium:

gallium alloy onto a conductive element.

A further object is to provide a novel electrolyte which is especially useful in electroplating an indium-gallium alloy onto a conductive element.

Still another object is to provide an electrolyte, and a can be operated at a temperature sufficiently high that I the indium-gallium alloy is plated in a molten state.

An important object is to provide an electrolyte, a

ethod for its preparation, and a method for electrodepositing an indium-gallium alloy upon a conductive element, which method and solution are especially useful in the commercial fabrication of microalloy rectifying connections.

In accordance with my invention, the foregoing objects are achieved by providing a novel electrolytic solution, a novel method for its fabrication, and a novel method for the electrodeposition of an indium-gallium alloy. -More particularly, the solution in accordance with my invention is one which is produced by heating a mixture consisting of a halide of indium, a halide of gallium, a conductivityenhancing salt and a solvent consisting principally of glycerol at a temperature less than the boiling point of said glycerol. My novel electrochemical method comprises the steps of immersing a conductive element in a heated solution of the above-mentioned substances, and.

applying to the conductive element a'potential negative with respect to that of an electrode also immersed in the solution by an amount exceeding the deposition potentials in said solution of both indium and gallium ions.

Generally speaking, my novel method of preparing my solution may take one of twopreferred forms. In one form a mass of glycerol preferably containing not more than about five percent by weight of water is heated to a temperature which typically may be of the order of 135 C. to 145 G. Then, while the mass of glycerol is maintained at a temperature within the latter range, a halide of trivalent indium, a halide of trivalent gallium, and a halide of ammonium are dissolved in succession into the mass of glycerol. In a second preferred form of my novel method, the mass of glycerol is admixed at room temperature with the aforementioned halides of indium, gallium and ammonium; next the mixture is heated to a temperature within a first range, e.g. 135 C. to 145 C., and is maintained at this temperature for a predetermined time, e.g. about ten minutes; the mixture is then heated to a higher temperature Within a second range, e.g. 158 C. to 162 C., and is maintained at a temperature within the latter range for a second predetermined time, e.g. five minutes. In each instance the mixture is preferably agitated while being heated.

Typically, the halides mentioned above may consist respectively of indium trichloride, ammonium chlorogallate, either in a pure form or admixed with ammonium chloride, and ammonium chloride. 7

In practicing my electrodeposition process, the deposited indium and gallium are derived from the aforementioned salts containing them, while the ammonium chloride serves primarily to enhance the conductivity of the electrolyte, thereby to increase the rate of deposition, and is also useful in fiuxing the metal upon which the alloy is being electrodeposited. Where such a solution.

is used, it is found that electrodeposition of an alloy containingan appreciable amount of gallium, e.g. of the order of 0.1 to 1.0 percent by weight, may be carried out at relatively moderate and commercially usable current densities, e.g. 15 to 50 amperes per square centimeter. This moderation of the current density needed to plate an indium-gallium alloy is achieved in accordance with my invention by reason of a substantial rise in the viscosity of the electrolyte which occurs when glycerol is heated in the presence of either indium or gallium ions. This increase in viscosity is believed to occur by reason of the condensation polymerization of glycerol, a reaction promoted by heat and catalyzed in the presence of indium and gallium ions. The exact natureof this polymerization reaction is not wholly understood. In such a viscous solution, the mobility of the indium and gallium ions during electrolysis is substantially decreased as compared to the mobility exhibited, for example, in glycerol which has not been heated in the presence of these ions. Because of this decrease in mobility, a considerable increase in the concentration polarization of indium at the cathode occurs because of the depletion of indium ions in the vicinity thereof. As a result, an equilibrium is established at the cathode favorable to the deposition of gallium ions, even though the latter ions have a single-deposition potential substantially higher than that of indium. Consequently, at relatively moderate current densities, it is feasible to deposit an indium-gallium alloy containing about 0.1 to about one percent by weight of gallium.

Other advantages and features of the invention will become apparent from a consideration of the following detailed description of specific examples thereof. In particular, five examples of preferred forms of my process for preparing my novel electrolyte are set forth hereinafter, after which a preferred form of the electrodeposition method of my invention is described.

Example 1 tween 75 and 81 percent by weight of ammonium chlorogallate, the remainder being ammonium chloride. The latter ingredients are stirred vigorously for about five minutes, any lumps of salts present thereinpreferably being broken up. The mixture of the salts and glycerol is then heated to a temperature between about C. and about C. When such a temperature has been attained, the solution is maintained thereat for a period ofabout ten minutes and is preferably stirred vigorously during this time. Next the solution is heated to a temperature between about 158 C. and about 162 C. After the solution has attained a temperature in the latter range, it is maintained thereat for about five minutes. Again the solution is preferably stirred vigorously during this heating period.

The temperature of the solution is then permitted to drop to about 120 C. and the solution is preferably filtered, e.g. through a fritted glass filter delivering its filtrate into a flask connected to a vacuum line.

Example 2 The amount of ammonium chlorogallate-ammonium chloride mixture in the bath need not be limited to the above figure of 4.8 grams per 800 grams of glycerol. On the contrary, where greater amounts of gallium are desired, a larger amount of the gallium-containing salt may be used, whereas Where lesser amounts of gallium in the alloy are desired a smaller amount of this salt is employed. For example, reliable plating has been obtained where the amount of this salt mixture has been as low as 3.2 grams and as high as 16 grams per 800 grams of glycerol. arriount-of ammonium chlorogailate-ammonium chloride In fabricating electrolytes containing an' mixture falling withinthis range, the same manipulative steps are preferably used as 'those described in Example 1 Example 3 'to the glycerol, the latter being stirred constantly and maintained at a temperature withinthe above-defined range. -'In practice, this dissolution of indium, trichloride requires about five'minutes. Preferably the next step is not takencuntilall of the indium trichloride has,

dissolved into the glyceroL. The indium trichloride having been dissolved, about two grams of a mixture typically containing about 81.8 percent of ammonium chlorogallate and about 18.2 percent by weight of ammonium chloride is stirred into the glycerol-indium trichloride mixture. As in the preceding steps, said mixture is maintained at a temperature between about 135 C. and about 145 C. The time required to dissolve this salt totally into the mixture is about three minutes. Next, about grams of ammonium chloride is stirred into the mixture the temperature of which is again maintained within the above-defined temperature range. Because the dissolution of ammoniumchloride in the foregoing mixture constitutes a markedly endothermic reaction, it is desirable to observe the temperature closely during this step and to take precautions to maintain it within the above-defined range. Typically 14 minutes are required to perform this last step. Finally, the resultant mixture is cooled to about 90 C; and is stored in an inert dry container, e.g. a polyethylene bottle.

Example 4 As in the case of Example 2, it is not necessary that the weight -of the ammonium chlorogallate-ammonium chloride mixture in the electrolyte be precisely two grams per hundred grams of glycerol. For example the bath works satisfactorily even where the mass of the salt mixture is as low as 0.4 gram per hunded grams of glycerol. In preparing solutions having an amount of the ammonium chlorogallate-ammonium chloride salt mixture within this range, the procedure of Example 3 may be employed.

Example 5 The substances forming the bath, as set forth in any one of the preceding examples, arefirst thoroughly admixed at room temperature. The temperature of the mixture'is then raised to a temperature within the range of about 135. C. and about 145 C. Once having been established at such a temperature, the mixture is maintained thereat for at least minutes, the mixture preferably being stirred constantly. Thereafter the mixture is cooled and stored as describedhereinbefore.

After my novel solution has been prepared by using any one of the above novel methods, it may then be used to plate upon a conductor an alloy containing principally indium and between 0.1 and 1.0 percent by weight of gallium. For example, where this process is used to prepare wire leads for microalloy transistors, this conductor typically will have the form of a wire between about one and two mils in diameter, composed of high purity nickel or a nickel alloy having the trade name Gridnic-T, or platinum-ruthenium. Before plating this whisker wireby utilizing my novel process, the wire is preferably tho-roughly cleansed. Such a cleansing may be provided by' first degreasing the wire in a vapor degreaser utilizing for example the solvent trichloroethylene. Next the wireis fired in ahydrogen atmosphere at 750 C.,

thereby to deoxidize it thoroughly. Thereafter, niclrel and Gridnic-T wires are immersed for 15 seconds in a boiling solution of concentrated hydrochloric acid satur ated with ammonium chloride; platinum-ruthenium wires may be cleansed in the same solution by-immersing them forabouttwo minutes. The wire is then rinsed numerous times, eg. 15 times, in distilled water, then is rinsed in absolute methanol and finally is air dried.

.Topractice the plating process according to my in vention, the wire thus cleansed is immersed in a bath prepared in one ofthe manners hereinbefore described and established at atemperature between about C. and about C. Also immersed in the same bath is an anode composed of an inert substance. Typically, this anode maybe a rod of spectrographically pure carbon or tungsten about one-eighth inch in diameter. A

low-impedance voltage source is then connected between the carbon rod and the wire in a manner such that the wire is established at a negative potential exceeding the deposition potentials in said solution of indium and gallium. In a specific embodiment, this voltage has a value of 15 volts. In addition it' is preferable that an atmosphere of dry'a'rgon or drynitrogen be passed over the surface of the bath during the plating process to prevent the bath from acquiring moisture from the atmosphere during the operation. 'By this electroplating operation, a prolate spheroidal globule of indium-gallium alloy is plated in molten-form upon the immersed end of the wire. After the globule has attained a size sufiicient for' use in fabricating the microalloy transistor in accordance with the Williams process described in the above-identified patent application, e.g. a prolate spheroidal globule having a major axis of about eight mils and a minor axis of about four mils, the plating is discontinued. Typically only a few seconds are required for such a globule to form.

Although the temperature of the plating bath is below the melting point of the indium-gallium alloy, it-is found that the alloy deposits on the wire in a molten" state because of the considerable amount of heat generated by the vigorous electrochemical action at the wire cathode. In this regard, it has been found t-hat'the average current density at the cathode ranges between about 15 and 50 amperes per square centimeter.

It is to be understood that the foregoing specific examples are given for the purpose of complete definiteness only and that it is not intended to limit the invention thereto. More particularly, while in each instance specific numerical values have been given for baths which are particularly useful for forming a solder to be used in manufacturing the microalloy transistor, it is clear that where alloys of different compositions are desired, these salts may be employed in other proportions, both as to absolute value and relative to one another. -Moreover, and' ag'ain with respect to the specific baths described above, it is not essential that the ammonium chlorogallate and ammonium chloride in the above-described salt mixture be present in the proportions specifically set forth. In this regard, the ammonium chlorogallateammonium chloride mixtures described above were selected merely because they represent mixtures which are readily available commercially. However," the use of an ammonium chlorogallate-ammonium chloride salt mixture containing substantially any other proportions of these salts is equally feasible, or for that matter pure ammonium chlorogallate may be employed. In this regard, it is desirable only that the ammonium chlorogallate appear in the specific form of plating baths described above in an amount of between about 0.0031 and abou 0.016 gram per gram of glycerol.

Similarly, the ammonium chloride may appear in the latter baths in proportions other than those noted. Pref-- erably it should appear in an amount adjusted with re-. spect-to the composition of the ammonium chlorogallateammonium ch oride mixture so that the total ammoniumv assigns ture lies in the range of about 0.00095 to 0.0050 gram per gram of glycerol.

In addition, it is not necessary thatany of the salts be chlorides. Alternatively, any or all of these salts may be any one of the other halides, i.e. fluorides, chlorides, brornides or iodides, or any combination thereof In this regard, it is desirable only that the indium salt be present in an amount such that the concentration of trivalent indiu'rn ions in the solution is about 0.073 gram per grain ofglycerol, the concentration of trivalent gallium ions is between about 0.00095 to 0.0050 gram per gram of glycerol, and the concentration of ammonium ions is about 0.036 gram per gram of glycerol.

Moreover the'conductivity-enhancing salt need not be ammonium chloride but may alternatively be any other salt having a high solubility, a high conductivity, and no tendency to cod eposit with indium and gallium. For example, tetramethyl ammonium chloride may be used, as may sodium chloride or potassium chloride, or other halides having cations of high activity. Ammonium chloride is advantageous, however, in that it is both highly soluble, highly conductive, and serves as a fiuxing agent.

In practicing the electroplating process, it is not essential that the plating voltage be volts as described above. For example, the plating voltage may lie anywhere between 7 and 30 volts. It is found that the higher the plating voltage, the more rapid the deposition of indiumgallium alloy upon the cathode. Moreover, the higher the applied plating voltage, the higher the concentration of gallium electrodepositing. Accordingly the plating voltage may be used as a means for adjusting the precise amount of gallium codeposited with the indium.

Lastly, it is to be understood that the cathode need not be composed of one of the metals specifically set forth above but may be composed of any metal or a suitable 1. The method of preparing an'electrolyte for use in,

the electrodeposition of an indium-gallium alloy, said method comprising the step of: heating a mixture consisting of a halide of trivalent indium, a halide of trivalent gallium, a conductivity-enhancing salt and a'solvent consisting essentially of glycerol at a temperature at least equal to the'boiling pointof water and less than the boiling point of glycerol.

ing said solvent at a temperature within saidra'nge, dis solving therein indium trichloride, ammonium chloro gallate, and ammoniumchloride.

4. The method of preparing an electrolyte for use in the electrodepositi'on of an indium-gallium alloy, said method comprising the steps of: admixing a halide of trivalent indium, a halide of trivalent gallium, a halide of ammonium, and a solvent consisting essentially of glycerol, in respective proportions such that there is produced in said solvent trivalent indium ions in a concentration of about 0.073 gram per gram of said solvent, trivalent gallium ions in a concentration of betweenabout 0.00095 to 0.0050 gram per gram of said solvent and ammonium ions in a concentration of about 0.036 gram per gram of said solvent; and while agitating said mixture, first heating said mixture to a temperature within a range of about 135 C. to about 145 C., next maintaining said mixture at a temperature within said range for about ten minutes, then heating said 'mixture to a temperature within a range of about 158 C. to about 162 C., and then maintaining said mixture within said last-named range for about five minutes.

5. The method of preparing an electrolyte for use in the electrodeposition of an indium-gallium alloy, said method comprising the steps of: stirring together the salts indium trichloride, ammonium chloride, and a salt mixture consisting of ammonium chlorogallate and ammonium chloride, with a solvent consisting of glycerol containing not more than 5 percent by weight of water, in proportions such that said salts are present in said solvent iii-substantially the following amounts: Indium trichloride0.14 gram per gram of said solvent, Said salt mixture-an amount providing between about 00031 and about 0.016 gram of ammonium chlorogallate per gram of said solvent,

Ammonium chloride-an amount sufiicient to establish the total concentration of ammonium chloride in said solvent at about 0.10. gram per gram of solvent,

thereby to form a second mixture; and while stirring said second mixture, first heating said second mixture to a temperature in the range of about 135 C. to about 145.

range of about 135 C. to about 145 C.; and while maintaining said mass at a temperature in said range, dissolving first in said mass a halide of trivalent indium in an amount producing in said mass a concentration of trivalent indium ions of about 0.073 gram per gram of said mass, dissolving next in said mass a halide of tri valent gallium in an amount producing in said mass a 2.*The method of preparing an electrolytefor use in the electrodeposition of an indium-gallium alloy, said method comprising the steps of: establishing a solvent consisting essentially of glycerol at a temperature in the range of at least the boiling point of water and less than the boiling point of glycerol, and while maintaining said solvent at a 'tmperatut'e within 'said range, dissolving successively therein a halide of trivalent indium, a halide of trivalent gallium, and a conductivity-enhancing salt.

3. The method of preparing an electrolyte for use in the electrodeposition of an indium gallium alloy, said method comprising the steps of: establishing a solvent 7 consisting of glycerol containing no more than about five percent by weight oi waterat a temperature in the range of about 135 C. to about 145 C.; and while maintainconcentration of trivalent gallium ions of between about 0.00095 and about 0.0050 gram per gram of said mass, and dissolving finally in said mass a halide of ammonium in an amount producing a total concentration of ammonium ions of about 0.073 gram per gram of said mass.

7. The method of preparing an electrolyte for use in the electrodeposition of an indium-gallium alloy, said method comprising the steps of: establishing a mass of glycerol at a temperature in the range of about C. to about C.; dissolving into said mass :1 halide of trivalentindium, a halide of trivalent gallium and a' mass, trivalent gallium ions in a concentration between about 0.00095 and about 0.0050 gram per gram of "said S mass, and ammonium ions in a concentration of about 0.036 gram per gram of said mass; and maintainingsaid glycerol at a temperature in said range for at least 20 minutes after adding the above-named salts thereto.

8. The method of preparing an electrolyte for use in the electrodeposition of an indium-gallium alloy, said method comprising the steps of: establishing a mass of glycerol at a temperature between about 135 C. and about 145 C.; and while maintaining said mass at said temperature, dissolving therein indium trichloride, ammonium chloride and a mixture of ammonium chlorogallate and ammonium chloride in about the following amounts:

Indium trichloride0.l4 gram per gram of said mass,

Said mixture-an amount providing between about 0.003 and about 0.016 gram of ammonium chlorogallate per gram of said mass,

Ammonium chloridean amount sufiicient to establish the total ammonium chloride concentration in said mass at about 0.1 gram per gram of said mass.

9. The method of electroplating indium and gallium simultaneously upon a conductive element, comprising operating said conductive elements as an electrolytic cathode in a solution of a halide of trivalent indium, a halide of trivalent gallium and a conductivity-enhancing salt in a solvent consisting essentially of glycerol, said solution being heated to a temperature between the respective boiling points of water and glycerol.

10. The method of electroplating indium and gallium simultaneously upon a conductive element, comprising immersing said conductive element in a solution consisting of a halide of trivalent indium, a halide of trivalent gallium, and a halide of ammonium in a solvent consisting essentially of glycerol and heated to a temperature between the respective boiling points of water and glycerol, and applying to said element a potential which is negative with respect to that of an inert electrode which is also immersed in said solution and has a value exceeding the respective values in said solution of the deposition potentials of said indium and gallium.

11. The method of electroplating indium and gallium simultaneously upon a conductive element, comprising immersing said conductive element in a solution consisting of indium trichloride, ammonium chlorogallate and ammonium chloride and a solvent consisting essentially of glycerol and heated to a temperature between the respective boiling points of water and glycerol, and applying to said element a potential which is negative with respect to that of an inert electrode which is also immersed in said solution and has a value exceeding the respective values in said solution of the deposition potentials of said indium and gallium.

12. The method of electroplating indium and gallium simultaneously upon a conductive element, comprising immersing said conductive element in a solution heated to a temperature between about 135 C. and about 145 C. and consisting of indium trichloride, ammonium chlorogallate, and ammonium chloride in glycerol containing not more than five percent-by weight of water, and applying to said element a potential which is negative with respect to that of an inert electrode which is also immersed in said solution and has a value exceeding the respective values in said solution of the deposition potentials of said indium and gallium.

13. The method of electroplatingindium and gallium simultaneously upon a conductive element, comprising immersing said conductive element in a solution heated to a temperature between about 135 C. and about 145 C. and consisting of a halide of trivalent indium, a halide of trivalent gallium, a halide of ammonium, and glycerol containing not more than five percent by weight of water, said halides of indium, gallium and ammonium being present in said glycerol in respective amounts such that there is produced in said glycerol trivalent indium ions in a concentration of about 0.073 gram per gram of said glycerol, trivalent gallium ions in a concentration between about 0.00095 and about 0.0050 gram per gram of said glycerol, and ammonium ions in a concentration of about 0.036 gram per gram of said glycerol; and applying to said element a potential which is negative with respect to that of an inert electrode also immersed in said solution and which has a value exceeding the respective values in said solution of the deposition potentials of said indium and gallium.

14. The method of electroplating indium and gallium simultaneously upon a conductive element, said method comprising the steps of: preparing an electrolyte by establishing at a temperature in the range of about 135 C. to about 145 C. a predetermined mass of glycerol containing no more than about five percent by weight of water, and while maintaining said mass at a temperature within said range and agitating said mass, dissolving first into said mass substantially anhydrous indium trichloride in the amount of substantially 0.14 gram per gram of said mass, dissolving next into said mass a salt mixture consisting of ammonium chlorogallate and ammonium chloride in an amount providing between about 0.003 and about 0.016 gram of ammonium chlorogallate per gram 'of said mass, and dissolving finally into said mass ammonium chloride in an amount providing a total ammonium chloride concentration in said mass of substantially 0.10 gram per gram of said mass; immersing said conductive element into said electrolyte; and while maintaining said electrolyte at a temperature within said range applying to said element a potential which is negative with respect to that of an inert electrode also immersed in said solution and which has a value producing at said element an average current density of between about 15 and about 50 amperes per square centimeter.

15. The indium-gallium electroplating solution produced by a process comprising the step of heating at a temperature between about 135 C. and 145 C. a mixture consisting of a halide of trivalent indium, a halide of trivalent gallium, a halide of ammonium and a solvent consisting essentially of glycerol, in respective amounts such that one gram of said solvent contains about 0.073 gram of trivalent indium ions, between about 0.00095 gram and about 0.0050 gram of trivalent gallium ions, and about 0.036 gram of ammonium ions.

References Cited in the file of this patent UNITED STATES PATENTS 2,458,839 Dyer et al. Jan. 11, 1949 2,582,376 Frary Jan. 15, 1952 2,582,377 Brown Jan. 15, 1952 2,582,378 Brown Jan. 15, 1952 FOREIGN PATENTS 711,065 Great Britain June 23, 1954 OTHER REFERENCES Transactions of The Electrochemical Society, vol. (1949), pp. 88-97.

Transactions of The Electrochemical Society, vol. 1953), pp. 126-130.

Transactions of The Electrochemical Society, vol. 66 (1934), pp. 107-115. 7

Modern Electroplating, John Wiley & Sons Inc., New York (1953), edited by A. G. Gray, pp. 87 and 88. 

14. THE METHOD OF ELECTROPLATING INDIUM AND GALLIUM SIMULTANEOUSLY UPON A CONDUCTIVE ELEMENT, SAID METHOD COMPRISING THE STEPS OF: PREPARING AN ELECTROLYTE BY ESTABLISHING AT A TEMPERATURE IN THE RANGE OF ABOUT 135* C. TO ABOUT 145*C. A PREDETERMINED MASS OF GLYCEROL CONTAINING NO MORE THAN ABOUT FIVE PERCENT BY WEIGHT OF WATER, AND WHILE MAINTAINING SAID MASS AT A TEMPERATURE WITHIN SAID RANGE AND AGITATING SAID MASS, DISSOLVING FIRST INTO SAID MASS SUBSTANTIALLY ANHYDROUS INDIUM TRICHLORIDE IN THE AMOUNT OF SUBSTANTIALLY 0.14 GRAM PER GRAM OF SAID MASS, DISSOLVING NEXT INTO SAID MASS A SALT MIXTURE CONSISTING OF AMMONIUM CHLOROGALLATE AND AMMONIUM CHLORIDE IN AN AMOUNT PROVIDING BETWEEN ABOUT 0.003 AND ABOUT 0.016 GRAM OF AMMONIUM CHLOROGALLATE PER GRAM OF SAID MASS, AND DISSOLVING FINALLY INTO SAID MASS AMMONIUM CHLORIDE IN AN AMOUNT PROVIDING A TOTAL AMMONIUM CHLORIDE CONCENTRATION IN SAID MASS OF SUBSTANTIALLY 0.10 GRAM PER GRAM OF SAID MASS, IMMERSING SAID CONDUCTIVE ELEMENT INTO SAID ELECTROLYTE, AND WHILE MAINTAINING SAID ELECTROLYTE AT A TEMPERATURE WITHIN SAID RANGE APPLYING TO SAID ELEMENT A POTENTIAL WHICH IS NEGATIVE WITH RESPECT TO THAT OF AN INERT ELECTRODE ALSO IMMERSED IN SAID SOLUTION AND WHICH HAS A VALUE PRODUCING AT SAID ELEMENT AN AVERAGE CURRENT DENSITY OF BETWEEN ABOUT 15 AND ABOUT 50 AMPERES PER SQUARE CENTIMETER. 